Technical Field
[0001] The present invention relates to a hydrogen storage laminated material, and more
particularly, relates to a hydrogen storage laminated material having excellent hydrogen
storage capability.
Background Art
[0002] With growing interest in hydrogen energy systems in recent years, research and development
of materials of the metallic alloys for hydrogen storage have been actively conducted
searching for materials for use as a hydrogen storage and transport medium, or for
use in energy conversion, separation and refinement of hydrogen gas, and the like.
The most important property as the metallic alloys for hydrogen storage is excellent
hydrogen storage capability. In the conventional materials, the atom ratio of stored
hydrogen to metal (H/M) is as follows: H/M = 1.00 for LaNi
5 and CaNi
5; H/M = 1.33 for Mg
2Ni; and H/M = 1.50 for ZrV
2.
[0003] In the case where the hydrogen storage material is a massive (bulk) state, the hydrogen
storage material is pulverized as a result of repeated hydrogen absorption-desorption
cycles. This pulverization significantly hinders the practical use as a hydrogen storage
material. Therefore, attempts have been made to form the hydrogen storage material
into a thin film that is less susceptible to pulverization. However, the hydrogen
absorption amount is reduced as compared to the massive sample. Moreover, in order
to use the hydrogen storage material for the electrode materials of the nickel-hydrogen
secondary batteries or the like, development of a material having H/M = 1.50 or more
as a standard of the hydrogen absorption amount has been expected.
[0004] Then, in order to solve these problems, a technology was disclosed in which the hydrogen
storage capability is improved by a laminated structure of Ti having an hcp structure
(group 4A metal, alloy, compound) and Cr having a bcc structure (group 6A, 7A, 8A
metal, alloy, compound) (Japanese Laid-Open Publication No. 9-59001). A material having
this laminated structure allows for significant improvement in the hydrogen storage
capability.
[0005] According to the above-mentioned laminated material, H/M of 1.5 or more can be easily
achieved, and under the good conditions, H/M of about 3.0 is also possible. However,
the following problems arise when the above-mentioned elements are used:
1) A relatively expensive metal is used. Ti is relatively commonly used, but is restricted
in terms of resources, and therefore becomes expensive when used in applications such
as batteries. In other words, utilization in large quantities is difficult industrially.
2) The weight is increased. The increased weight is highly disadvantageous for portable
use or the like.
[0006] It is an object of the present invention to provide a hydrogen storage laminated
material having an increased H/M value and capable of achieving reduction in weight
and of being mass-produced industrially.
Disclosure of Invention
[0007] A hydrogen storage laminated material of the present invention has a laminated structure
of first and second layers, wherein the first layer is formed from an alloy or compound
including an element of a group 2A or 3A or an element of at least one of the groups
2A and 3A, and at least partially includes a body-centered cubic structure, and the
second layer is formed from an alloy or compound including an element of one of groups
6A, 7A and 8A or an element of at least one of the groups 6A, 7A and 8A.
[0008] The inventors have confirmed for the first time that a laminated structure that is
durable as an industrial material can be obtained by laminating a layer having an
element of the group 2A, 3A and a layer having an element of the group 6A, 7A, 8A,
and that this laminated structure is light in weight and also has excellent hydrogen
storage capability. In the above-mentioned laminated material, significant reduction
in weight can be realized as compared to the hydrogen storage laminated material described
in the above-mentioned Japanese Laid-Open Publication No. 9-59001. Thus, this laminated
material can be made highly suitable as a main member of an apparatus intended to
be used in, e.g., applications in which rich resources as well as lightness in weight
are of great importance. In other words, this laminated material can be made highly
suitable as a hydrogen supply source for the hydrogen-utilizing fuel cells, a portable
or mobile hydrogen source, or a small hydrogen supply source provided inside and outside
the houses, business offices and the like, and thus, can be used as a safe, hydrogen-utilizing
power supply or heat source.
[0009] The group 2A element or group 3A element included in the first layer of this laminated
structure generally has a hexagonal close-packed (hcp) structure at ordinary temperature
and pressure, but the above-mentioned laminated structure at least partially includes
a bcc structure. The reason why the first layer including the bcc structure has the
increased hydrogen storage capacity can be considered as follows: unlike the conventional
idea, in the case where the first layer is changed to a crystal of the bcc structure
including an element of at least one of the groups 2A and 3A, the number of interstitial
sites capable of being occupied by hydrogen atoms is increased to at most nine per
atom of the group 2A or 3A element, as shown in Figs. 4A to 6B. Moreover, since it
is possible to control the interatomic distance of the first layer by changing the
interatomic distance and constituent element of the second layer, the bonding power
between the group 2A or 3A element and hydrogen as well as the size of the hydrogen
atom itself can be changed. Thus, hydrogen's moving speed and moving capability inside
the crystal, as well as the bonding power acting on the hydrogen atoms inside the
crystal can be adjusted, whereby the number of hydrogen atoms capable of being stored
per constituent atom of the first layer can be increased to at most nine. Furthermore,
since it is possible to control movement and diffusion of hydrogen, a material capable
of easily performing hydrogen absorption and desorption at 100°C or less, and preferably
80°C or less, can be made.
[0010] Since the above-mentioned hydrogen storage laminated material of the present invention
has such a multi-layer film structure, the hydrogen storage capacity that is significantly
superior to that of the conventional bulk hydrogen storage materials can be obtained.
[0011] The hydrogen storage laminated material of the present invention can be obtained
by laminating two different kinds of substances onto a substrate using, e.g., a vapor
phase method like a PVD (Physical Vapor Deposition) method such as vacuum deposition
method, ion plating method and sputtering method, and a CVD (Chemical Vapor Deposition)
method such as plasma CVD method. In addition to the method as described earlier,
in the case where the physical vapor deposition or chemical vapor deposition is conducted
in the atmosphere in which high-purity hydrogen gas is present, the bond distance
between atoms is increased as compared to the case where hydrogen is not present,
whereby the hydrogen storage capability is increased. This is desirably conducted
at the hydrogen gas pressure of 1 atm or less, and preferably, in the reduced-pressure
hydrogen atmosphere of 0.1 atm or less. Although the effect of the hydrogen gas is
not clear, the reason for this can be considered as follows: the hydrogen atoms are
taken in simultaneously with formation of the laminated structure, so that the bond
distance between the metal atoms resulting from the taking in of the hydrogen atoms
is automatically controlled to such a distance that is preferable for taking in and
out of hydrogen.
[0012] The following consideration is also possible: for example, a change in the electron
structure due to increase in the interface (increase in the number of interfaces)
or increase in the number of interface atoms resulting from a reduced lamination cycle
length of the first and second layers may be involved in the increased hydrogen storage
capacity. Therefore, in a preferred aspect of the present invention, the lamination
cycle, that is, the length of a unit lamination including the first and second layers
is repeatedly laminated.
[0013] By thus laminating the lamination structure repeatedly, the hydrogen storage capability
can further be improved.
[0014] Moreover, in a preferred aspect of the present invention, the second layer is formed
from a material having a bulk modulus that is larger than that of the first layer.
[0015] By laminating a layer including a group 6A, 7A, 8A element, which has a bcc structure
at the ordinary temperature and pressure and also has a larger bulk modulus than a
layer including a group 2A or 3A element, and the layer including a group 2A or 3A
element, a bcc structure becomes likely to be produced in the first layer. In other
words, a metal or the like forming the first layer and having an hcp structure at
the ordinary temperature and pressure is subjected to elastic deformation at the interface
with the second layer due to the high bulk modulus of the second layer, and becomes
susceptible to phase transition to the bcc structure at the interface or in the inside
of the first layer.
[0016] Moreover, in the above-mentioned hydrogen storage laminated material, it is desirable
that the laminated material has lattice distortion produced therein.
[0017] With the lattice distortion produced in the laminated material, the bcc structure
is likely to be produced within the first layer, and particularly at the interface.
As a result, the hydrogen storage capability can be improved.
[0018] More desirably, in the above-mentioned hydrogen storage laminated material, the first
layer includes a group 2A element, Mg, as a main element.
[0019] Mg has small specific gravity, and therefore is highly advantageous for reduction
in weight. Mg is also rich in resources, and is suitable for industrial mass production.
Accordingly, the hydrogen storage laminated material can be used in large quantities
in applications in which a reduced weight is important, while maintaining high hydrogen
storage capability.
[0020] In the above-mentioned hydrogen storage laminated material, it is highly desirable
that the second layer includes a group 8A element, Fe, as a main element.
[0021] Fe is outstanding as an inexpensive industrial material. The hydrogen storage laminated
material using Fe can be mass-produced at low cost, and therefore can be made highly
suitable as an electrode material for the nickel-hydrogen secondary batteries, a hydrogen
supply source for the hydrogen-utilizing fuel cells, a portable or mobile hydrogen
source, or a small hydrogen supply source provided inside and outside the houses,
business offices and the like. As a result, this hydrogen storage laminated material
can be advantageously used as a new, alternative energy source to the fossil fuel.
In particular, combination with a multi-layer material including Mg as a main element
in the first layer meets weight and economical requirements, and therefore is highly
desirable.
Brief Description of Drawings
[0022] Fig. 1 is an external view showing the process of a film forming apparatus.
[0023] Fig. 2 is a cross sectional view schematically showing the structure of a laminated
film in an embodiment of the present invention.
[0024] Fig. 3 is a schematic diagram showing the structure of an apparatus for realizing
hydrogen storage treatment.
[0025] Figs. 4A and 4B are diagrams showing the sites of hydrogen atoms in an fcc lattice.
Fig. 4A is a diagram showing octahedral interstitial sites (O-sites), and Fig. 4B
is a diagram showing tetrahedral interstitial sites (T-sites).
[0026] Figs. 5A and 5B are diagrams showing the sites of hydrogen atoms in a bcc lattice.
Fig. 5A is a diagram showing octahedral interstitial sites (O-sites), and Fig. 5B
is a diagram showing tetrahedral interstitial sites (T-sites).
[0027] Figs. 6A and 6B are diagrams showing the sites of hydrogen atoms in an hcp lattice.
Fig. 6A is a diagram showing octahedral interstitial sites (O-sites), and Fig. 6B
is a diagram showing tetrahedral interstitial sites (T-sites).
Best Mode for Carrying Out the Invention
[0028] Hereinafter, a laminated structure formed by an ion plating method will be described
as one embodiment of the present invention. Any one of the group 2A elements Be, Mg,
Ba, Ca and the group 3A elements Y, La, Yb was used as a metal element forming a first
layer, and a group 6A element, Cr, or a group 8A element, Ni, was used as an element
forming a second layer.
[0029] A laminated film of the first and second layers was formed by the ion plating method
using vacuum arc discharge. In this case, Cr or Ni forming the second layer has a
bulk modulus that is larger than that of the above-mentioned element forming the first
layer. A specific method for making this multi-layer film is described in conjunction
with Fig. 1.
[0030] Fig. 1 is an external view showing the structure of a film forming apparatus. Referring
to Fig. 1, cathode evaporation materials (evaporation sources 6 and 7) of the elements
forming the first and second layers are disposed in a vacuum vessel 1, and a substrate
4 is mounted onto a substrate holder 3 provided on a rotary table 5. The substrate
4 is formed from, for example, silicon. After sufficient evacuation, the rotary table
5 is rotated while evaporating the cathode evaporation materials by arc discharge
in vacuum or in the atmosphere of argon gas. Thus, the element forming the first layer
is formed into a film on the substrate when it faces the evaporation source 6 of the
first layer, whereas the element forming the second layer is formed into a film when
facing the evaporation source 7 of the second layer.
[0031] The respective thicknesses (lamination cycle) of the first and second layers were
adjusted by controlling the rotational speed of the rotary table 5. An example of
the typical conditions for making the laminated structure using various elements is
as shown in Table 1 below:
Table 1
Arc Current (First Layer) |
80A |
Substrate Bias |
- 50V |
Arc Current (Second Layer) |
80A |
Substrate |
Si |
Film-Forming Pressure (Torr) |
≦ 0.01mTorr |
Table Rotational Speed |
1-30rpm |
[0032] From the above Table 1, in the typical example, the respective arc currents of the
evaporation source 6 of the first layer and the evaporation source 7 of the second
layer were 80A each; the film-forming pressure was 0.01 mTorr or less; the substrate
bias was -50 V; and the rotational speed of the table was 1 to 30 rpm.
[0033] Fig. 2 shows a cross sectional view of the laminated film of the first and second
layers thus obtained.
[0034] Referring to Fig. 2, for example, a Be layer 6a and a Cr layer 7a are successively
repeatedly laminated on the silicon substrate 4 to form a laminated film 10. In Fig.
2, T denotes the thickness (nm) of the lamination cycle.
[0035] The above-mentioned laminated film was subjected to hydrogen storage treatment by
an electrolytic charge method. An apparatus for conducting the hydrogen storage treatment
is shown in Fig. 3.
[0036] Referring to Fig. 3, in conducting the hydrogen storage, the sample 10 shown in Fig.
2 was soaked in a 0.1 M-NaOH solution and a Pt counter electrode 12 was soaked in
a 0.5 M-K
2SO
4 solution. A negative current was applied to the sample 10, whereas a positive current
was applied to the Pt counter electrode 12, both for a predetermined time period by
means of a constant-current power supply 11. TR6120A made by Advantest was used as
the constant-current power supply 11. Note that the current value was basically 10
mA, and the current application time was set to one hour. A value as given by current
(A) x time (s) corresponds to the quantity of electricity, and this value was used
to calculate the hydrogen generation amount by the electrolysis (Faraday's law). The
hydrogen charge conditions were common to all the laminated materials. Measurement
of stored hydrogen was conducted with EMGA621 made by Horiba. This apparatus is capable
of conducting any one of hydrogen absolute quantity analysis and temperature-programmed
analysis.
[0037] Present Examples Nos. 1 to 16 and Comparative Examples 25 to 30 as shown in Table
2 below were subjected to the above-mentioned hydrogen storage treatment. The result
is shown in Table 2.
Table 2
|
Material Combination |
Lamination Cycle (nm) |
Hydrogen Storage Capacity (H/M) |
XRD Peak Showing bcc Structure |
Lattice Distortion |
Present Example |
1 |
Be/Cr |
5 |
2.0 |
Exist |
Exist |
2 |
Be/Ni |
5 |
2.5 |
Exist |
Exist |
3 |
Mg/Ni |
5 |
2.5 |
Exist |
Exist |
4 |
Mg/Cr |
5 |
2.5 |
Exist |
Exist |
5 |
Ba/Ni |
5 |
2.0 |
Exist |
Exist |
6 |
Ba/Cr |
5 |
2.5 |
Exist |
Exist |
7 |
Y/Ni |
5 |
3.0 |
Exist |
Exist |
8 |
Y/Cr |
5 |
3.0 |
Exist |
Exist |
9 |
La/Ni |
5 |
3.0 |
Exist |
Exist |
10 |
La/Cr |
5 |
3.0 |
Exist |
Exist. |
11 |
Yb/Ni |
5 |
2.5 |
Exist |
Exist |
12 |
Yb/Cr |
5 |
2.5 |
Exist |
Exist |
13 |
Ca/Ni |
5 |
2.5 |
Exist |
Exist |
14 |
Ca/Ni |
50 |
1.5 |
Exist |
Exist |
15 |
Ca/Cr |
5 |
2.5 |
Exist |
Exist |
16 |
Ca/Cr |
50 |
1.5 |
Exist |
Exist |
Comparative Example |
25 |
BeNi5 |
---- |
1.0 |
None |
---- |
26 |
MgNi5 |
---- |
1.5 |
None |
---- |
27 |
CaNi5 |
---- |
1.0 |
None |
---- |
28 |
LaNi5 |
---- |
1.0 |
None |
---- |
29 |
BeCr5 |
---- |
1.0 |
None |
---- |
30 |
LaCr5 |
---- |
0.5 |
None |
---- |
[0038] Lattice Distortion: a lattice constant was calculated from an XRD peak showing bcc,
and presence/absence of distortion was determined from comparison with a lattice constant
of Cr or Ni.
[0039] Note that, in Present Examples 1 to 16, two lamination cycles (T in Fig. 2) of 5
nm and 50 nm were used, and the element of the second layer is limited to Cr (group
6A) or Ni (group 8A) while changing the element of the first layer among the group
2A elements Be, Mg, Ba, Ca and the group 3A elements Y, La, Yb. Moreover, bulk materials
(single-layer structure) of BeNi
5, MgNi
5, CaNi
5, LaNi
5, BeCr
5 and LaCr
5 were used as Comparative Examples 25 to 30. Presence/absence of a diffraction peak
of X-ray diffraction due to a bcc structure from the first layer is also shown in
Table 2.
[0040] Note that, specifically, the hydrogen storage capacity was obtained by the following
method:
[0041] First, the film (or laminated film) is warmed up, and hydrogen leaving the film is
quantified by gas analysis. Subsequently, the film having discharged hydrogen is dissolved
in acid, and the film atoms are quantified by chemical analysis. H/M was obtained
from both results. Here, in Table 2, presence/absence of lattice distortion was determined
from comparison between a lattice constant of a bcc structure produced in the first
layer and a lattice constant of a bcc structure of the second layer. With the lattice
distortion being present, the first layer is subjected to elastic deformation at the
interface due to the metal or the like forming the second layer having a high bulk
modulus, and thus is forced to have a bcc structure.
[0042] The result of Table 2 shows that, by laminating the above-mentioned combinations
of the elements, the laminated material according to the present invention has hydrogen
storage capability that is significantly higher than that of the conventional bulk
materials. Moreover, it can be appreciated that the laminated material of the present
invention is lighter in weight than any one of the conventional hydrogen storage materials
regardless of whether it is a bulk material or laminated material, and is capable
of being mass-produced industrially. Accordingly, the laminated material can be made
highly suitable as a hydrogen supply source for the hydrogen-utilizing fuel cells,
a portable or mobile hydrogen source, or a small hydrogen supply source provided inside
and outside the houses, business offices and the like, and thus, can be used as a
safe, hydrogen-utilizing power supply or heat source.
[0043] Then, the laminated materials were also tested in which the element forming the first
layer is limited to Mg or Ca while changing the element forming the second layer among
Mo, Mn, Fe and W. More specifically, samples 51 to 58 were made by the same method
as that of Table 2, and were caused to store hydrogen by the hydrogen storage method
shown in Fig. 3 and then examined for the hydrogen storage capacity. The result is
shown in Table 3.
[0044] The result of Table 3 shows that a layer including Ca (group 2A) or Mg (group 2A),
which enables significant reduction in weight, can be used as the first layer. Moreover,
Ca or Mg is rich in resources, and can be mass-produced industrially. Regarding the
hydrogen storage capability, it can be appreciated that lamination of a layer including
Ca or Mg and a layer of any one metal element of the groups 6A, 7A and 8A results
in highly excellent hydrogen storage capability, as shown in Table 3. All the samples
of Table 3 have H/M of 2 or more, which is well beyond 1.50, the value required for
the electrodes of the nickel-hydrogen secondary batteries, and therefore meet the
weight and economical requirements. These results have enabled an electrode material
of the nickel-hydrogen secondary batteries which is inexpensive and capable of being
supplied in large quantities to be realized in applications in which a reduced weight
is important.
Table 3
|
Material Combnation |
Lamination Cycle (nm) |
Hydrogen Storage Capacity (H/M) |
XRD Peak Showing bcc Structure |
Lattice Distortion |
Present Example |
51 |
Ca/Mo |
5 |
2.5 |
Exist |
Exist |
52 |
Ca/Mn |
5 |
2.5 |
Exist |
Exist |
53 |
Ca/Fe |
5 |
2.5 |
Exist |
Exist |
54 |
Ca/W |
5 |
2.0 |
Exist |
Exist |
55 |
Mg/Mo |
5 |
2.5 |
Exist |
Exist |
56 |
Mg/Mn |
5 |
2.5 |
Exist |
Exist |
57 |
Mg/Fe |
5 |
2.0 |
None |
Exist |
58 |
Mg/W |
5 |
2.5 |
Exist |
Exist |
[0045] Note that it can be readily supposed that the laminated material is not necessarily
formed from a single element, and the same effects can be expected from a compound,
alloy, or the like. However, the above results of Tables 2 and 3 show that the hydrogen
storage capacity that is superior to that of the conventional materials can be obtained
even by merely laminating a layer including an element of the group 2A or 3A and a
layer including an element of the group 6A, 7A, 8A.
[0046] The hydrogen storage laminated material according to the present invention has a
laminated structure of two or more different kinds of substances, wherein one layer
thereof is formed from a substance including an element of the group 2A or 3A and
at least partially includes a bcc structure, and the other layer is formed from a
substance including at least one element of the group 6A, 7A and 8A. By using this
hydrogen storage laminated material, significant reduction in weight as well as industrial
mass production as compared to the conventional hydrogen storage materials can be
achieved while assuring high hydrogen storage capability. Moreover, the hydrogen storage
laminated material of the present invention can be made highly suitable as a high-sensitive
hydrogen sensor, and also as a hydrogen supply source for the hydrogen-utilizing fuel
cells, a portable or mobile hydrogen source, or a small hydrogen supply source provided
inside and outside the houses, business offices and the like, and thus, can be used
as a safe, hydrogen-utilizing power supply or heat source.
[0047] The embodiments as disclosed herein are by way of illustration and example only in
every respect, and are not to be taken by way of limitation. The scope of the present
invention is defined by the appended claims rather than the above description, and
includes all modifications within the sense and scope equivalent to the definition
of the appended claims.